The present invention relates to the field of portable appliances operated from batteries, and more particularly to the field of electronic musical instruments such as electric guitars. While the great majority of guitars have been fitted with only passive electronic circuits, there has been a trend to incorporate active on-board battery-powered preamplifiers and audio processors to overcome the shortcomings and performance compromises inherent in passive guitar electronics. This trend, as part of a general proliferation of battery-operated appliances, has created requirements, heretofore unsatisfied, for positive, accurate and convenient determination and indication of battery condition.
One solution sometimes found on scientific instruments and the like has been the use of a built-in analog or digital voltmeter, operable by a selector switch or pushbutton. However such a solution is unnecessarily costly, complex, inconvenient and confusing to non-technical users such as musicians. Other known battery-testing schemes often conflict with aesthetics, particularly in retrofitting an existing product design, where styling constraints may be imposed precluding drilling the enclosure or adding exterior fittings of any kind, ruling out the use of known systems requiring switches, pushbuttons or exterior-mounted indicators. Known battery-testing schemes requiring manual operation of pushbuttons or switches tend to be disregarded by the user because of the inconvenience of having to perform the test; consequently, unexpected battery failures are not uncommon.
Needs have developed for indicating the condition of a pair of batteries used in a dual power supply configuration, and for disconnecting both of them from their loads, controlled by a single pair of contacts, which may be part of an audio jack mounted on an electric guitar, where the contacts are actuated by removal of the plug of an audio cable from the jack.